1. Field of the Invention
This invention relates to surface-coated cemented carbide articles and more particularly, it is concerned with surface-coated cemented carbide articles or parts, for example, for cutting tools having hard coating layers, capable of resisting severer cutting conditions.
2. Description of the Prior Art
It is well known in cutting tools that the temperature of a tool edge is 800.degree. C. or higher during cutting. As the temperature of an edge becomes higher, a cemented carbide such as WC-Co deforms by heat to lower the flank wear resistance. Various surface-coated cemented carbides having hard coating layers have thus been used so as to improve the cutting property, but the effects of these hard coating layers are not sufficient and the above described problem has not been solved. Thus, the life of such a tool is often shortened by deformation due to heat of the cemented carbide as a substrate.
When a cemented carbide is coated with a hard layer, the strength of an edge is lowered as compared with the cemented carbide as a substrate and thus the tool tends to be broken and can hardly exhibit high effectiveness for any of many kinds of workpieces such as of steel or cast iron.
The coating layers can be divided broadly into two categories: (1) single layers of TiC, TiCN or TiN and composite layers thereof, which will hereinafter be referred to as "Ti compound system" and (2) coating layers comprising an Al.sub.2 O.sub.3 layer as the outermost layer, which will hereinafter be referred to as "Al.sub.2 O.sub.3 system". Typical of the coating of Ti compound system are those described in Japanese Patent Publication No. 24982/1976 which discloses a film structure comprising an inner layer of TiC and an outer layer of TiN, or an inner layer of TiC, an intermediate layer of TiCN and an outer layer of TiN such that the nitrogen content is gradually increased from the inside to the outside, the film structure having both an excellent rake surface wear resistance and flank wear resistance. In the coating layer structure of this kind, however, the substrate of a cemented carbide tends to deform by heat generated during cutting, for example, in the case of heavy cutting or high speed cutting and the effects by coating hard layers are not sufficiently exhibited in wear resistance.
In general, when the wear of a tool is increased to some extent, the cutting resistance is increased, thus resulting in rapid progress of wearing and breakage of the edge, so that the life of the tool becomes unstable. Lately, stabilization of the tool life has been required for the purpose of elimination or reduction of labor in cutting operation and there has been a tendency of exchanging a tool for another when the flank wear width reaches approximately 0.1 to 0.15 mm, although in the prior art, a tool was generally used until reaching a flank wear width of about 0.2 to 0.3 mm. The flank wear width of approximately 0.1 to 0.15 mm has been called the initial flank wear width which will hereinafter be referred to as "initial wear" and of late, this initial wear resistance has been considered as an important factor to govern the tool life.
On the other hand, Al.sub.2 O.sub.3 -coated cemented carbides, for example, as disclosed in Japanese Patent Publication No. 13201/1977, show an excellent wear resistance in high speed cutting because of the excellent heat resistance and nonreactivity with steels of Al.sub.2 O.sub.3, but have a problem of breakage resistance because of the brittleness of Al.sub.2 O.sub.3 itself.